The present invention relates in general to self-propelled center pivot irrigation systems which are comprised of a plurality of self-propelled articulated span units which mount interconnecting sections of water carrying conduit, each such span unit having drive motor means and motor control means which regulate the operation of the drive motor means to maintain the span units in pre-determined relative alignment. In particular, the present invention relates to the control means which regulate the operation of the drive motor means.
The preferred relative alignment for the individual span unit of the system is accomplished when the units are in essentially a straight longitudinal array with the individual units revolving about the center pivot of the system aligned in an imaginary true vertical plane revolving about an imaginary truely vertical line located co-axial with the system center pivot. "True Vertical" as used herein is the indication of vertical as provided by a so called "spirit" level or a plumb-bob suspended on a plumb line, i.e., truely vertical as to the gravitational center of the earth.
Heretofore, it has been common in such articulated systems to connect a sensor arm to one span unit and to have the sensor arm operationally interact with a motor control actuator mechanism of an adjacent span unit. It has been found in these prior embodiments that the use of a rigidly mounted sensor arm will measure changes in angularity in a plane perpendicular to the vertical plane of its unit only and thus transmit spurious signals to the motor control actuator mechanism when the self-propelled irrigation system traverses uneven terrain. Specifically, when the span units are travelling either uphill or downhill, there is a rotational motion relative to the longitudinal axis of the span units as well as a vertical displacement of span units relative to adjacent span units. This rotative motion of the span units and vertical displacement of the span units have caused prior motor controls to sense changes in angularity relative to a false vertical and thus transmit signals to the drive motor control which were not related to the desired relative alignment of the units in the true vertical plane. Rather, these prior sensors have given erroneous control directions causing over misalignment and the systems even to shut down.